The field of this invention is the area of therapeutic nanoscale particulate compositions, in particular to formulations of improved solubility and stability for the delivery of tissue factor. These formulations can be used to kill tumors, to stop bleeding, as topical hemostatic agents and as reagents in prothrombin time assays.
Tissue factor (TF) is the integral membrane protein that triggers blood coagulation. TF is composed of two fibronectin type 3 domains, a single membrane-spanning domain, and a short cytoplasmic domain (FIG. 1A). TF is typically expressed on the cell surface. A type I integral membrane protein, TF has its N-terminus located outside the cell and its C-terminus is in the cytoplasm.
TF is abundant in adventitial cells, found exterior to the smooth muscle of blood vessels. This layer can be considered a hemostatic envelope (Drake et al. 1989. Amer. J. Pathol. 134:1087-1097). Where there is damage to a blood vessel, TF participates in the clotting cascade to form a “patch” to stop further blood loss from the vasculature. Where blood vessels contain plaque and there is a rupture of the plaque, TF participates in the formation of a hemostatic “patch” at the point of rupture. This serves as a focus for clotting, leading to further occlusion of the blood vessel at that location.
TF functions to initiate blood clotting by selectively binding one of the soluble plasma proteins (factor VII or the activated form, factor VIIa) with high affinity. This results in the formation of TF:VIIa complexes on the cell surface. Factor VIIa, the first enzyme in the blood clotting cascade, is a serine protease that circulates as a soluble protein in the plasma. Factor VIIa is an extremely weak enzyme (low activity) unless it is bound to its protein cofactor (TF). Factor VIIa is allosterically activated when it binds TF, creating an extremely potent, two subunit enzyme (TF:VIIa). The TF:VIIa complex then triggers blood clotting by proteolytically activating two plasma serine protease zymogens (factors IX and X), which then go on to propagate the clotting cascade. The ultimate result is the formation of blood clots composed of polymerized fibrin and activated platelets. TF is thought to be involved in thrombotic diseases in addition to its beneficial role in preventing blood loss from the vasculature.
Structurally, TF is a type I integral membrane protein composed of an extracellular domain, a single membrane-spanning domain and a short cytoplasmic tail. TF must be incorporated into suitable phospholipid membranes in order to exhibit maximal activity. Soluble TF is thousands of times less active than TF embedded in a suitable membrane, underscoring the essential role of membrane anchoring for TF function. In order for TF to exhibit strong procoagulant activity, the membrane or disc in which it is embedded must contain negatively charged phospholipids, desirably phosphatidylserine. There are several methods available for incorporating purified TF into phospholipid vesicles and nanoscale disc-like structures of varying composition.
Nanoscale disc-like particles comprising a membrane scaffold protein (MSP, naturally occurring or engineered) and phospholipid have been successfully used to provide stable, soluble and biologically active hydrophobic proteins. See, for example, WO 02/40501 and US Published Applications 2004/0053384 and 2005/0182243 for a thorough discussion of these particles, the structural proteins in them and their formation. These particles contain the phospholipid in the form of a disc which is surrounded by a “belt” formed of the amphiphilic membrane scaffold protein (MSP). Where there is a hydrophobic protein incorporated, it is bound in, within or to the phospholipid portion and may or may not have peripheral association with the encircling MSP. These particles are typically from about 5 to about 50 nm, usually about 5 to about 20 nm, in diameter, depending on the specific composition.